Gas discharge lamps are being considered for use in automobiles by automotive designers so as to allow for lowering the hood lines of the automobile in order to improve the appearance and aerodynamic performance of the automobile. Gas discharge lamps such as a xenon lamp, metal halide lamp and xenon-metal halide lamp do not include filaments, but instead typically include two electrodes inside a quartz enclosure filled with selected ingredients. It is preferred that the selected ingredients include xenon, which provides for the instant light needs of the automobile. During a start mode, or warm-up mode, of the lamp, an arc is created across the electrodes for generating light. After the start mode, the lamp is then sequenced into a run mode wherein it is operated continuously. In both the start mode and the run mode, the lamp's arc must be controlled to avoid instability which could degrade performance and life of the lamp during operation. The instability is primarily manifested by a bowed arc condition between the electrodes of the lamp.
In an automotive application of a gas discharge lamp, the power requirements for the lamp may vary, for example, from about 30 watts during the run mode to about 110 watts during the start mode. The transition from the start mode to the run mode results in an equivalent lamp resistance which varies widely.
Accordingly, the ballast for operating the lamp must be effective for operating over such range of output power while avoiding instability of the arc.
For providing the increased voltage needed for effectively operating the gas discharge lamp, a ballast in the form of a DC-to-DC converter connected in circuit with the usual automotive battery can be used. One type of DC-to-DC converter usable for this purpose is the tapped-inductor boost converter. A conventional tapped-inductor boost converter includes a pair of input terminals capable of being connected to a DC power source serving as its input voltage, a pair of output terminals for providing an output voltage greater than the input voltage to a load, a tapped-inductor having first and second windings and a tap between said windings, an active switch connected to the tap and cyclically operable with a varying duty cycle for controlling current flow through the first winding and thereby variably controlling the output voltage, a passive switch for controlling current flow through the second winding, and a filter capacitor for controlling the ripple component of the output voltage.
The term "active switch" as used herein is intended to denote a switching device, the conduction of which is controlled by an external signal. The term "passive switch" as used herein is intended to denote a switching device, the conduction of which ceases when its current reaches zero or reverses polarity. The active switch typically includes a transistor, and the passive switch typically include a diode.
A conventional tapped-inductor boost converter would be unacceptable for use in operating a gas discharge lamp such as a metal halide lamp, since the resistance of the metal halide lamp varies during the start and run modes, and the output ripple current of the conventional converter would hinder the desired operation of the discharge lamp. More specifically, during the run mode, a lamp current with a relatively high percentage ripple content is desired for maintaining arc stability of the lamp; and during the start mode, a lamp current with a relatively low percentage ripple content is desired for maintaining arc stability. But in a conventional boost converter applied to operate a gas discharge lamp, the converter develops in its output current a percentage ripple content opposite to that which is desired, with a relatively high percentage ripple content being developed during the start mode, and a relatively low percentage ripple content being developed during the run mode due to the dynamics of the capacitive type output filter which is typically present. Furthermore, in a conventional boost converter the capacitive type output filter operates in a manner which could possible damage the lamp during the transition period from start to run.
In the operation of a gas discharge lamp in an automotive application, a relatively high output power ranging from about 30 watts to about 110 watts, for example, is required from the converter. Operation at this relatively high power tends to impose a relatively large voltage stress across the active switch arranged within the converter. This stress is created by the energy stored in the leakage inductance of the tapped-inductor being discharged into the active switch when the active switch is rendered non-conductive, or opened. A relatively expensive active switch, therefore, would have to be used to accommodate the voltage stress, which would result in a more expensive converter. Alternatively, conventional snubbers could be used to dissipate the energy imposed upon the active switch by discharge of the leakage inductance, but such snubbers are typically inefficient and reduce the overall efficiency of the ballast significantly.